1. Field of Invention
The present invention relates to a pad layout method for a circuit board and a circuit board structure thereof, more particularly to a pad layout method for a surface mount circuit board and a surface mount circuit board thereof, for preventing the surface mounted component and the circuit board from being warped or deformed.
2. Related Art
Plastic raw materials are approximately classified into thermoplastic plastics and thermosetting plastics. Recently, the plastic material commonly adopted in the electrical connector industry is the thermoplastic plastic. The thermoplastic plastic, usually in a particle form under the room temperature, is capable of being changed into a melted status after being heated to a certain temperature, injection-molded after being cooled, and softened into the melted status after being heated again, and then injection-molded again. Therefore, the thermoplastic plastic can be repeatedly injection-molded through being heated and melted.
However, usually, when a surface mount technology (SMT) process is performed on an electrical connector of an elongated shape, such as an electrical connector applied in a dual in-line memory module (DIMM) or an electrical connector applied in a single in-line memory module (SIMM), due to the elongated size, the electrical connector of the memory module may be softened after being baked on the circuit board by a fan convection or an IR-reflow soldering oven.
In order words, during the SMT process, the hot air released from the fan convection or the IR-heating device is usually above the connector of the memory module, such that an upper portion of a plastic body of the connector is affected with a higher temperature. Relatively, a lower portion of the plastic body is affected with a lower temperature. Different temperatures affected at the upper and lower portions of the plastic body may easily cause the plastic body to have a larger thermal expansion rate at the upper portion than that at the lower portion, or to have a larger thermal expansion rate at the lower portion than that at the upper portion, thereby causing a central portion of the plastic body to be arched upward or downward, and further causing the connector structure to be warped and deformed.
In such a manner, the plastic body may easily generate the phenomenon of non-uniform expansion degree due to non-uniform heating, and generate deformation on the structure (that is, the plastic body is warped into a U shape or an inversed U shape), such that the pin terminal inserted in the plastic body departs from the pad on the circuit board. In order words, the pin terminal and the pad on the circuit board that are original aligned may fail to practically contact each other or even generate a misalignment phenomenon due to the different expansion amounts of different portions of the plastic body, so that the pad cannot be accurately soldered to the pin terminal, thereby causing the problem of missing solder or insufficient solder. Moreover, the holding intensity of soldering is also insufficient, or even severer problems exist, such as a short circuit caused by soldering the pin terminal to a wrong position of the pad of the circuit board. Therefore, it is necessary to seek for a preventing method of solving such a problem of non-uniform thermal expansion rate.